Rob.Analog

What Happens when a Video Source is Disconnected from the ADV7180, ADV7182 and ADV728x

Discussion created by Rob.Analog on Apr 25, 2017
Latest reply on Apr 25, 2017 by Rob.Analog

This FAQ relates to the ADV7180, ADV7182, ADV7280, ADV7280-M, ADV7281, ADV7281-M, ADV7281-MA, ADV7282, ADV7282-M and ADV7283. These parts will be referred to as the "ADV7180/ADV728x" in this post. 

 

There are some analog video sources that can stop outputting Horizontal synchronization (Hsync) and vertical synchronization (Vsync) information (e.g. VCRs and poor quality cameras). The ADV7180/ADV728x parts were designed to be robust to these poor video sources.

 

When a video source is disconnected from the ADV7180/ADV728x parts the ADV7180/Monza will enter a coast mode. In this mode the ADV7180/Monza will assume that the source has stopped outputting synchronization signals and will output whatever signal is on the analog input pins (most likely noise). The ADV7180/Monza will try and output the same horizontal and vertical timing signals as the last valid input it had before the video source was disconnected. So during coast mode the ADV7180/Monza will output noise for a period of time.

 

How long the ADV7180/ADV728x remains in the coast mode depends on which mode it has been programmed into. The ADV7180/ADV728x will remain in coast mode for over 1 second when it has been programmed with standard scripts. The ADV7180/ADV728x will remain in coast mode for approximately 250 ms when it has been programmed with fast switch scripts. See this link for more information on ADV7180 fast switch scripts. See this link for more information on ADV728x fast switch scripts.

 

There is no register bit or interrupt that can be used to reliably show that the ADV7180/ADV728x has entered coast mode. The INST_HLOCK (user map register 0x13 bit[0]) can show that the ADV7180/ADV728x has lost lock to a video source but it is not reliable (i.e. it can give false positive and false negative results).

 

After the timeout period the ADV7180/ADV728x will realize that the video source is not connected. The IN_LOCK, LOST_LOCK and FSC_LOCK bits (User map register 0x10 bits [0:2]) will toggle and if programmed the interrupt pin will toggle. The ADV7180/ADV728x will also enter free-run mode and output a blue screen.

 

It should be noted that discontinuities can occur when:

  • The video source is disconnected and the part enters coast mode.
  • The ADV7180/ADV728x goes from cost mode to free-run mode (i.e. from outputting noise to outputting a blue screen).
  • The ADV7180/ADV728x goes from free-run mode to active mode (i.e. when a video source is reconnected).

 

During each of these discontinuities the ADV7180/ADV728x can sometimes output fields of video that are longer than standard. This can confuse the devices that are receiving the output from the ADV7180/ADV728x. This can cause memory buffers in the receiver parts to become overloaded. In order to prevent this the receiver part should implement a line counter or similar algorithm to ensure that the memory does not get overloaded.

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